How to Read Corneal Topography and Tomography: Hard-Core Tips

Last updated: October 2026

Corneal topography and tomography decide who gets LASIK, who needs cross-linking and which toric lens goes into an eye. This page gives the practical rules for reading Placido and Scheimpflug (Pentacam-type) maps without being fooled by artifacts.

Know your machine

• Placido-based topographers measure anterior curvature with high resolution, but cannot see the posterior surface or measure thickness. They depend heavily on a smooth tear film.
• Scheimpflug tomographers (e.g., Pentacam) measure anterior and posterior elevation and full pachymetry. They are essential for refractive screening.
• Anterior segment OCT adds epithelial thickness mapping, which can reveal early keratoconus that front-surface curvature hides.

Acquisition: garbage in, garbage out

• Check the quality indicator (for example, Pentacam "QS: OK") on every scan. Repeat any scan that is not acceptable.
• Tear film: ask the patient to blink just before capture. Dry eye and MGD create irregular mires and false irregular astigmatism; treat the ocular surface and rescan.
• Avoid scanning immediately after drops, applanation tonometry or gonioscopy, which disturb the epithelium.
• Contact lens holiday: lens wear warps the cornea. Common practice is to stop soft lenses for at least 1–2 weeks and rigid lenses for several weeks, and to repeat scans until two are stable.
• Take at least two good scans per eye and check that they agree before making a decision.

Choose the right map

• Axial (sagittal) curvature: smooth overall picture of refractive power. Good for general screening and astigmatism patterns.
• Tangential (instantaneous) curvature: shows true local steepness. Best for locating a cone and the edge of a refractive ablation.
• Elevation maps: show how far the surface sits above or below a reference sphere. These are the most sensitive maps for ectasia, especially the posterior elevation.
• Pachymetry map: shows the thickness profile and the location of the thinnest point.
• Scales: use a fixed absolute scale for screening and for comparing scans. Auto-normalised scales exaggerate small, normal irregularities and make healthy corneas look abnormal.

Patterns on curvature maps

• Round, oval and symmetric bow-tie patterns are normal variants.
• Asymmetric bow-tie with inferior steepening or a skewed radial axis (SRAX) raises suspicion of keratoconus.
• "Crab-claw" / "kissing doves" on the curvature map, with a band of inferior thinning about 1–2 mm above the limbus, suggests pellucid marginal degeneration.
• Rabinowitz indices (Placido): central K over 47.2 D and an inferior-superior (I-S) value over 1.4 D are suspicious. The KISA% index combines these; values above 100% suggest keratoconus.
• Contact lens warpage can mimic keratoconus with irregular steepening, often superior. It improves after lens removal; true ectasia does not.

Elevation maps: where ectasia shows first

• Read elevation against a best-fit sphere (BFS), usually over the central 8 mm.
• Posterior elevation often becomes abnormal before the front surface or the visual acuity changes.
• Commonly used rules of thumb at the thinnest point: anterior elevation above about +12 µm and posterior elevation above about +15 µm are suspicious; posterior elevation above about +20 µm strongly suggests ectasia. Interpret these together with the full pattern, not in isolation.
• An isolated "island" of elevation over the thinnest point is more worrying than a symmetric ridge pattern (which is typical of regular astigmatism).
• The Belin/Ambrósio enhanced ectasia display recalculates the reference sphere while excluding the area around the thinnest point. A cone stands out much more clearly on the enhanced map. Its final "D" value combines several parameters; values above about 1.6 are flagged as suspicious.

Pachymetry: more than one number

• A thin cornea on its own is not keratoconus, and a normal thickness does not exclude it. The distribution of thickness matters more.
• Look at the location of the thinnest point (displaced inferotemporally in keratoconus) and the difference between the thinnest point and the centre.
• In a normal cornea, thickness increases steadily from the thinnest point to the periphery. A faster-than-normal increase (abnormal pachymetric progression) suggests ectasia.
• Compare the two eyes. Significant asymmetry in thinnest point thickness or pattern is suspicious.
• Epithelial masking: the epithelium thins over a cone and thickens around it (a "doughnut" pattern on epithelial maps), smoothing the front surface. This is why a normal Placido map can hide early keratoconus.

Refractive surgery screening

• Check that the topographic astigmatism matches the manifest refraction in magnitude and axis. A mismatch suggests lenticular astigmatism, irregular cornea or a poor-quality scan.
• Always combine tomography with the clinical picture: age, family history, eye rubbing, allergy, and stability of refraction.
• Percent tissue altered (PTA): PTA of 40% or more is a recognised risk factor for ectasia after LASIK.
• When in doubt, prefer surface ablation, a phakic IOL, or no surgery. Ectasia is far harder to treat than to prevent.

Toric IOL and cataract planning

• Posterior corneal astigmatism is usually against-the-rule. Relying on front-surface keratometry alone tends to overestimate with-the-rule and underestimate against-the-rule astigmatism. Use total corneal power or a toric calculator that accounts for it.
• Compare keratometry from the biometer and the topographer. If magnitude or axis disagree significantly, look for ocular surface disease and repeat.
• Irregular astigmatism on the map (from scars, keratoconus or pterygium) can make a toric IOL perform poorly.
• After refractive surgery, standard keratometry is unreliable. See our post-refractive IOL calculation page and IOL calculators.

Following keratoconus and cross-linking

• Use the same device under the same conditions for every visit.
• A rise in Kmax of 1 D or more over about a year is a commonly used marker of progression, but Kmax is noisy. Confirm with repeat scans and look at posterior elevation and pachymetry too.
• Progression in children and teenagers is often faster, so review them more often. See our keratoconus page.